Fix mem_plus_reg predicate

[MaryBennett]

Issues #73 #74

mem_plus_reg needs to check that both operands are registers. Added constraint to prevent operands (mem (plus reg reg)).

[MaryBennett]

| Category             | Previous  | Current  |       Delta |
| :------------------- | -------------: | ------------: | ----------: |
| Expected passes      |          19113 |         19141 |         +28 |
| Unexpected failures  |             173 |           173 |           - |
| Unexpected successes |              - |             - |           - |
| Expected failures    |              6 |             6 |           - |
| Unresolved testcases |              6 |             6 |           - |
| Unsupported tests    |            1268 |           1268 |          - |

[MaryBennett]

Reviewed on CORE-V GCC call 13 Oct